1. Field of the Invention
The present invention relates generally to differential pressure flow sensors for use in very low pressure flow lines (with low flow rates) and that provide a signal when a differential pressure exceeds a predetermined mount and in particular relates to a differential pressure flow sensor utilizing multiple layers of flexible plastic membranes mounted on a support member across a central passageway in loosely fitting superimposed relationship with respect to each other to provide a discontinuity and allow a predetermined bleed flow rate therethrough. An electrically conductive strip of flexible material is placed in juxtaposed relationship with the multiple layers of flexible plastic membrane. The conductive strip has a normally closed switch portion completing an electrical circuit between first and second contacts and which is opened when the flow rate in the central flow passageway exceeds the allowable bleed flow rate capacity through the multiple plastic membranes thus providing an indication of an open conductive path.
2. Description of the Prior Art
As indicated in U.S. Pat. No. 5,155,471, there are many different types of prior art rupture disk monitoring devices and systems in which, when a disk ruptures, an electrical indication is given. However, as pointed out in the patent, it is a problem to obtain a reliable break limit as well as an indication of rupture disk failure in very low pressure flow situations.
In order to overcome that problem, U.S. Pat. No. 5,155,471 discloses a burst disk sensor employed in a relief flow line adjacent a low pressure rupturable disk which is damped normally upstream of the sensor across the flow line by means of inlet and outlet head members. The sensor has a basic frame or gasket member that can be damped between the inlet and outlet head members. The gasket carries a very thin membrane of plastic film thereon having the flow way portion slit to provide flexible petals. The petals are maintained closed across the flow way by means of one or more conductors bonded thereon and continuous thereacross, each of such conductors having a point of minimum break strength located centrally of the flow way cross section. The resistance of the membrane is thus eliminated from the break strength equation as only the conductor minimum breakpoint senses the fluid flow and consequent flow pressure presence, such as the result of an upstream rupture of a rupture disk. Electrical indication and/or alarm may then be actuated in response to the conductor breakage or open circuit.
Further, in U.S. Pat. No. Reissue 34,308, there is disclosed a flow sensor device in which a conductive loop is either secured to or passes in close proximity to a seal. If the seal is ruptured, it breaks the loop and thus provides a signal that the rupture has occurred.
Another known rupture disk monitor simply comprises a holder with a conductor extended across the flow passageway and contacting an electrical circuit to form a closed circuit. Upon rupture of the device, one end of the conductor is simply moved out of contact with another end thus breaking the circuit. The unit can be reset by simply reinserting the one end of the conductor into a conductive slot on the other end so as to re-establish the conductive path.